A Singular Case of Prophage Complementation in Mutational Activation of recET Orthologs in Salmonella enterica Serovar Typhimurium

Abstract

A class of mutations that suppress the recombination defects of recB mutants in Salmonella enterica serovar Typhimurium strain LT2 activates the normally silent recET module of the Gifsy-1 prophage. Allele sbcE21 is a 794-bp deletion within the immunity region of the prophage. Concomitant with activating recET, sbcE21 stimulates Gifsy-1 excision, resulting in unstable suppression. Early studies found both recB suppression and its instability to depend on the presence of the related Gifsy-2 prophage elsewhere in the chromosome. In cells lacking Gifsy-2, the sbcE21 allele became stable but no longer corrected recB defects. Here, we show that a single Gifsy-2 gene is required for Gifsy-1 recET activation in the sbcE21 background. This gene encodes GtgR, the Gifsy-2 repressor. Significantly, the sbcE21 deletion has one end point within the corresponding gene in the Gifsy-1 genome, gogR, which in strain LT2 is a perfect duplicate of gtgR. The deletion truncates gogR and places the Gifsy-1 left operon, including the recET and xis genes, under the control of the gogR promoter. The ability of GtgR to trans-activate this promoter therefore implies that GtgR and GogR normally activate the transcription of their own genes. Consistent with the symmetry of the system, a similar deletion in Gifsy-2 results in a Gifsy-1-dependent sbc phenotype (sbcF24). Two additional Gifsy-1 deletions (sbcE23 and sbcE25) were characterized, as well. The latter causes all but the last codon of the gogR gene to fuse, in frame, to the second half of recE. The resulting hybrid protein appears to function as both a transcriptional regulator and a recombination enzyme.